1. Field of the Invention
This invention relates to household coffee brewing apparatus and more particularly to such apparatus which provides an indication of sufficient calcification that cleaning is desirable or necessary.
2. Description of the Prior Art
Coffee brewing apparatus incorporating a throughflow heater of the kind to which the present invention relates is well known. Such a heater, which serves for electrically heating water to be passed to a brew station including a brew basket supporting a filter filled with fresh coffee grounds, is included in the flowthrough path of a water conduit extending from a water reservoir to the brew station.
The present invention is concerned with the safeguarding of such a throughflow heater from the effects of mineral deposits. Water pollution is a virtually universal problem, and in areas with so-called hard water, the deposition of minerals, primarily calcium carbonate, in apparatus in which tap water is heated to temperatures above 68.degree. C., is inevitable. Such deposition occurs on the inside of the water conduit, so that electrical heating coils customarily mounted on the outside, are thus able to transmit less and less heat to the water within the conduit owing to the heat insulating layer of the mineral deposits.
Therefore, such coffee brewing apparatus is commonly accompanied with instructions to the effect that, if hard tap water is used, the apparatus must be periodically cleansed, for example, with vinegar. In practice, such cleaning is often times not effected, or at least not on a timely or routine basis. In the long run, therefore, a thick layer of scale will have formed within the water conduit, with the result that the heating coils cannot dispose of their heat to the water, at least not to a sufficient extent, and become overheated and ultimately may burn out. Apart from being a potential fire hazard, this may cause permanent damage to the apparatus, possibly requiring its replacement.
In order to remedy this drawback, coffee brewing apparatus of the kind referred to have sometimes been provided with safety devices in the form of fuses connected in series with the heating coil. When a given temperature is exceeded, these break the circuit. Nevertheless there is the drawback in this instance that, before the circuit is broken, the heating coil may already have become so hot as to burn out, or the temperature may already have become so elevated that the coil loses its tension, thereby interfering with proper heat transfer in future operations.
Many prior proposals for safeguarding the heater mechanism of coffee brewing apparatus from overheating due to excessive mineral deposits in the water conduit and thereby resulting in damage to the mechanism and to the housing, have been based on the sensing of temperature in the space surrounding the heater and within the housing of the apparatus. Thus, it is known to provide a thermoswitch on the outside of the coil windings which, when a threshold temperature is exceeded, either actuates an alarm lamp or operates a re-settable switch which can be manually reinstated after cooling. However, these devices. did not usually indicate specifically that the inoperative condition resulted from excessive mineral deposits in the water conduit. In other instances, an indicator informing the operator of the need to clean the unit would be energized after an arbitrary period of time had elapsed or after the machine had been operated for an arbitrary number of brew cycles. Again, these expedients sometimes proved to be unreliable in actual practice.
A number of other expedients have been devised and patented. In the instance of U.S. Pat. No. 4,141,286 to Smit, for example, at least a portion of a flowthrough heater is made of transparent material enabling scale deposits to be visually observed by a user. U.S. Pat. No. 4,292,499 to Kleinschmidt et al discloses a calcification indicator in a system utilizing PTC resistors for heating. With one resistor element located at a water entry region and another located at a water exit region, electronic circuitry is provided for determining when a difference of current flow in the exit heating element compared to the entry heating element exceeds a given value indicative of the need for cleaning. An indicator is triggered when cleaning is required.
In U.S. Pat. No. 4,139,761 to Obrowski, a thermally responsive switch and calcification indicator are together provided electrically in parallel to a water heater and an associated heater thermostat. The thermally responsive switch is subject to opening at a temperature much higher than the thermostat such that when the heater thermostat opens, the thermally responsive switch remains closed. This enables the heater to continue to be energized but the calcification indicator to turn on to indicate that cleaning is desirable.
Another indicating system is disclosed in U.S. Pat. No. 4,214,148 to Fleischhauer which utilizes first and second temperature dependent switches in series with the heater resistor. The switching temperature of the first switch is below that of the second switch but both temperatures are in the range which occur with calcification in the water heating operation. A time delay member actuates an indicating lamp after a predetermined time but operates only when the first switch is open. The time delay is for the purpose of preventing a false indication which can occur due to temperature fluctuations in the heating operation. The time delay member ceases operation and returns to its original state when both switches are both either open or closed.
The foregoing systems were known to the applicants when they conceived the present invention. It was their intent to improve upon the reliability of the known systems by obtaining a more accurate indication of a calcified condition. At the same time, they sought to achieve this goal with a system exhibiting a simplified construction and operation resulting in its being less expensive to manufacture and maintain. They believe the present invention achieves all of these goals.